1. Field of the Invention
The present invention relates to an electron microscope and a method for observing microscopic images obtained thereby. More particularly, the invention relates to an electron microscope suitable for obtaining a distribution image of the intensity and direction of a magnetic field inside a specimen, and to a method for observing microscopic images obtained by that microscope.
2. Description of the Related Art
Conventionally, the so-called defocus Lorentz method is used to measure the internal magnetic field of a specimen composed of a ferromagnetic or antiferromagnetic material. The method is implemented through the use of a transmission electron microscope. In operation, the electron beam generated by the electron microscope is focused a little below or above the specimen so that the deflection of the electron beam within the specimen is obtained as a black-and-white contrast image. This method is illustratively described in a Japanese publication titled "Thin Films" by Kanehara and Fujiwara (Shokabo, 1979).
The electron holography method, as described in Physical Review, B25 (1982), 6799 and on, has recently become popular for similar observation purposes. This method involves observing electron waves transmitted through a magnetic specimen with respect to the interference with a reference electron wave.
Another method for observing the internal magnetic field of a magnetic material is about to be put to practical use. Depicted in Ultramicroscopy, 3 (1978), p. 203, this method is based on the so-called DPC-STEM (Differential Phase Contrast Scanning Transmission Electron Microscope). In operation, the electron microscope detects a deflection of a focused electron beam, the deflection being caused by the Lorentz force to which the beam is subjected when the latter passes through a specimen.
Of the above prior art methods, the defocus Lorentz method has the following disadvantages: One disadvantage is that because the focal point of the transmitted electron beam is shifted away from a specimen, the defocus Lorentz method fails to provide a distribution image of a magnetic field within the specimen at a sufficiently high level of resolution. Another disadvantage is that with the electron beam irradiated over the entire surface of the specimen, the micro-analysis of a specific spot on the specimen is not available.
The electron holography method is superior to the defocus Lorentz method in terms of resolution, but the former method still irradiates its electron beam over the entire surface of a specimen, making the micro-analysis of specimen spots impossible. On the other hand, the DPC-STEM method allows a focused electron beam to be directed at a particular target spot on a specimen, thus concurrently permitting micro-analysis of specimen parts. If the electron beam is sufficiently focused under the DPC-STEM method, a high level of resolution is obtained.
However, the DPC-STEM method is dogged by the problem of the so-called spot shift. That is, the incidence spot of the transmitted electron beam shifts in a detecting apparatus as the beam is made to scan a specimen. Meanwhile, the incidence spot shifts due to the Lorentz force stemming from the magnetic field inside the specimen. When the incidence spot shift becomes as large as the scan-caused spot shift, it is impossible to detect the magnetic field inside the specimen.